This invention relates to programmable logic, and in particular to a simple bias driver for a current source that has no feedback and provides good supply rejection.
As described in the incorporated patent applications, programmable logic devices include thousands of repeaters, buffers and logic blocks distributed across a fairly large semiconductor structure. Many of the elements making up various programmable logic circuits described in the co-pending patent applications use bipolar differential amplifiers.
FIG. 1 is a schematic diagram of one example of a conventional differential amplifier 10. Differential amplifier 10 is a common configuration designed to amplify a difference voltage between two input signals. Differential amplifier 10 includes two NPN transistors (T.sub.1 and T.sub.2), two collector resistors (R.sub.C1 and R.sub.C2) coupling collectors of the transistors to a first reference voltage, and a current source 12 coupling emitters of the transistors to a second reference voltage. The input signals are provided to the bases of transistors T.sub.1 and T.sub.2. An output taken at the collector of transistor T.sub.2 provides a voltage that depends upon the difference of the input signal voltages.
The prior art recognized that the use of current source 12 as a bias current provides differential amplifier 10 with a common-mode gain that is about zero. There are many types of possible current sources that could be used with differential amplifier 10.
FIG. 2 is a schematic diagram of one typical type of current source 12. Current source 12 includes an NPN transistor T.sub.3 and an emitter resistor R.sub.E. Resistor R.sub.E couples the emitter of transistor T.sub.3 to the second reference voltage. To control current source 12, a bias circuit must provide both a base bias current and a base bias voltage.
There are a number of drawbacks when using current source 12, or similar types of current sources. These drawbacks relate to bias signal distribution, size, and operation. The large size of typical programmable logic devices and the use of differential amplifiers across the entire semiconductor structure requires distribution of the bias current and the bias voltage. There are many well-known problems with distribution of bias signals, not the least of which is compensation of capacitive loading of long lead lines that interconnect the bias circuit and the current sources used with each of the thousands of differential amplifiers.
With respect to the size drawback, current source 12 includes transistor T.sub.3 and resistor R.sub.E. Together these elements require a relatively large amount of space on the semiconductor structure. When the number of current sources is large, the space required for each current source becomes significant.
Regarding the operational drawback, under particular conditions, transistor T.sub.3 can go into saturation. For a circuit integrated on a single semiconductor structure, it is undesirable for a bipolar transistor to operate in its saturation region as charge gets dumped into a substrate of the semiconductor structure. It is undesirable to dump charge into the substrate. The likelihood of a transistor going into saturation is increased when turning the transistor on and off. Programmable logic devices include the ability to turn various differential amplifiers on and off, typically by turning its associated current source on and off.